The present invention relates to a vehicle steering or suspension system, and particularly relates to a boot seal for sealing a movable joint in a vehicle steering or suspension system.
Boot seals are used to protect the bearings in joints between relatively movable parts of vehicle steering and suspension systems. The boot seals are typically formed of a thermoplastic or thermosetting elastomer. Examples of prior art elastomers are neoprene rubber, a blend of ethylene-propylene rubber and polypropylene marketed by Monsanto Company of St. Louis, Mo. under the trademark SANTOPRENE, and HYTREL polyester marketed by E.I. DuPont de Nemours Co.
The boot seals are typically blow molded into the configuration in which they are to be installed, and are snapped into place in an interference fit which is secured by metal clips.
Boot seal failure can be caused by fatigue, punctures, cuts or tears, and abrasive wear. Boot seal failures are a common cause of joint failure. If a boot seal fails, water and dirt can get into the joint, and/or grease can leak out. It is desirable to increase the resistance of boot seals to failure.
The present invention resides in a boot seal for use in sealing a joint. The joint comprises first and second relatively movable parts. The boot seal is in the form of a sleeve which extends between and around the vehicle relatively movable parts. The sleeve has a laminate wall which comprises at least two layers of thermoplastic elastomeric material which are bonded together. At least one of the layers is a stretch-toughenable polyester thermoplastic elastomer which has been diametrically stretched in an amount effective to increase the toughness of the one layer. The laminate wall has a percent elongation to rupture of at least 100, and a flexural modulus which is less than 100,000.
Preferably, the laminate wall has a puncture resistance of at least 50 Newtons/mm wall thickness.
Preferably, the stretch toughened one layer is a stretch-toughenable polyester material which is resistant to dimensional change when exposed to heat.
Preferably, the boot seal is a laminate of a first outer layer which is formed of a stretch-toughened polyester material and a second inner layer which is formed of a thermoplastic elastomer, wherein the outer layer polyester material has a generally higher degree of toughness and a greater resistance to hydrocarbon chemicals than the inner layer thermoplastic elastomer, and wherein the inner layer thermoplastic elastomer has a higher degree of flexibility and softness than the outer layer polyester thermoplastic material.
In an embodiment of the present invention, the inner layer thermoplastic elastomer is a polyolefin.
The present invention also resides in a process for making a boot seal. Two thermoplastic elastomeric raw materials are obtained and fed separately into an extruder. The raw materials are separately extruded from the extruder as coaxial tubular molten streams having an outside diameter D1 into a mold cavity comprising a corrugated inner wall having an inside diameter D2. The molten streams of thermoplastic elastomeric material are vacuum expanded against the mold cavity inner wall and then cooled to a semi-solid state while vacuum held against the mold cavity inner wall. The amount of expansion (D2/D1) preferably is in the range of about 200% to about 700%. One of the raw materials is a polyester thermoplastic elastomer which is stretch toughenable by said vacuum expansion and which when cooled has a percent elongation to rupture of at least 100 and a flexural modules less than 100,000 psi.
The boot seal preferably has a puncture resistance of at least 50 Newtons/mm thickness of the boot seal.
The thicknesses of the molten streams of thermoplastic elastomeric material are preferably controlled to achieve a boot seal wall thickness in the range of about 0.6 to about 2 mm.
In a preferred embodiment of the present invention, the mold cavity is defined by an endless series of movable molds, each mold having an open clamshell configuration prior to the point of extrusion, and a closed clamshell configuration after extrusion, the extrusion being continuous.